He began a long association with composerArthur Schwartz when they teamed up for the BroadwayrevueThe Little Show in 1929. They would continue to work on and off over the next 30 or so years. Dietz served in the US Navy in World War I and became editor of their magazine, Navy Life. During World War II, he assisted the U.S. Treasury Department with the publicity and promotion of War Bonds and created stage shows for the Coast Guard with composer Vernon Duke. He appears as a recurring character in the mystery novels of John Dandola which involve a sleuthing MGM publicity girl.

Dietz saved copies of every document relating to his career, as well as relating to the publicity campaigns of every MGM film he publicized. After his death, this vast trove of artifacts was donated to the New York Public Library for the Performing Arts. The archive on Dietz constitutes its single largest archive on any person or subject.

Dietz was married three times. He married Elizabeth Bigelow Hall in 1917, and they divorced in 1936. He married Tanis Guinness Montagu on January 25, 1937,[3] and had a daughter; they divorced 14 years later, in 1951. Later that year, he married the costume designer Lucinda Ballard. He died in New York City of Parkinson's disease.[4]